Loud noise when using environmental system

[MikeDelta]

Our aircraft is a 'straight' 690 with the Keith Freon kit installed.

Every time I switch on the factory air conditioning, a very loud noise can be heard. This happens when it is pressurized or not. However when pressurized, after reaching roughly 13000 ft (cabin set at 8000ft) the sound slowly fades and goes away eventually.
The previous owner told me that it could be a loose or damaged seal. It sounds like it is coming from the back of the cabin.

Has anyone ever a similar issue? I'm curious as to what the cause is and I don't really know where to look.

[donv]

By "factory" you mean the Keith system? Or the bleed air system?

[ghancock]

And by noise you mean bang, squeal... :-)

Glenn

[MikeDelta]

The factory unit is the regular ECU. Keith was STC'ed.

The sound is hard to describe. Can I post videos here?

Its like a piece of duct tape inside a tube through which air is blowing at high velocity.

[donv]

You could certainly post a link to the video.

Could it be that one of your air ducts is partially blocked, or alternatively has come loose? Do you get normal differential?